Air heater fuel control system



H. L. KIRK 2,979,124

AIR HEATER FUEL CONTROL SYSTEM Filed Sept. 29, 1958 April 11, 1961ELECTRONIC 92 90 l354 OVERHEAT SWITCH INVENTOR. a HAROLD L. KIRK .1519.3 y 94 W 6 96 AIR FLOW SWITCH ATTORNEY United btates Patent f 2,979,124AIR HEATER FUEL CONTROL SYSTEM Harold L. Kirk, Bettendorf, Iowa,assignor to American Air Filter Company, Inc., Louisville, Ky., acorporation of Delaware Filed Sept. 29, 1958, Ser. No. 764,070

11 Claims. (Cl. 158-28) This invention relates to a control system forspace heaters employing gasoline or a similar liquid fuel.

Hubbard US. Patent 2,758,591, issued August 14, 1956 illustrates aportable space heater of the type commonly used to supply heated air toaircraft and other enclosed spaces. The present invention is directed toan improved fuel control system for a heater of this general type.

Such heaters conventionally comprise: a fuel burnercombustion chamber; aheat exchanger; blower means for supplying combustion air andventilating air to the burner and heat exchanger respectively; fuelsupply and ignition means; and means for controlling the flow of fuel inaccordance with departures in sensed air temperature from a selected airtemperature.

Upon starting such a heater, the means controlling.

fuel flow ordinarily calls for or demands full or maximum fuel flow tothe burner since the sensed discharge air temperature is considerablybelow the desired discharge air temperature. The discharge of a maximumor relatively high rate of fuel into the burner when starting isobjectionable because of thermal shock to heater structure, inefficientcombustion and the possibility that a substantial quantity of fuel maybe discharged into the burner without ignition thereof, and otherreasons. While the effect of high initial fuel discharge may berelatively unimportant for a heater such as the commercial embodiment ofthe aforesaid Hubbard patent wherein the maximum rate of fuel into theburner is below g.p.h., a substantial problem arises with highercapacity heaters adapted to burn fuel at a rate of 25 g.p.h. or higher.

Thus, one object of this invention is to provide a control systemoperable to give a relatively low fuel discharge rate or a so-called lowfire start upon initiating burner combustion.

Another object is the provision of a control system operable to increaseprogressively the rate of fuel discharge into the burner from a low orminimum rate upon initiating burner combustion to a higher ratedetermined by normally controlling fuel flow control means.

Another object is the provision of such a control systeni includingmeans responsive to combustion conditions in the burner for controllingfuel flow to the burner.

Still another object is the provision of a control system operative toreset itself automatically to give a low fire start upon subsequentstarting after termination of fire in the burner for any reason.

Briefly, in accordance with one feature of the invention, a low firestart valve provided in parallel with a normally controlling fuel flowthrottle valve is arranged to be operated progressively from a fullyopen position upon firing the burner, to a fully closed position asfiring continues. Thus, upon firing the burner, fuel bypasses thenormally controlling throttle valve and thereby renders its control offuel flow ineffective. As the low fire start valve progressively closesin response to a continuing combustion condition, control is restoredprogressively to the throttle valve.

Further, the invention contemplates that the low fire start valve becontrolled in response to a condition of fuel flow or pressure in abypass line or conduit, this condition in turn being controlled by'combustion conditions in the burner or other conditions related to safeheater operation.

r 2,979,124 Ce Patented Apr. 11, 1961 The invention will be explained inconnection with the accompanying drawing illustrating a preferredembodiment by way of example, and'wherein:

Figure 1 is a diagrammatic view of a fuel system embodying the presentinvention;

Figure 2 is a sectional view of a low fire start valve suitable for usein the invention; and 1 Figure 3 is a schematic view of an electricalcircuit for.

controlling fuel conditions in the primary bypass line.

The combustion and air heating system is of the general type illustratedin Hubbard US. Patent 2,758,591, issued August 14, 1956, and as showndiagrammatically in Figure 1 comprises: forced air blower means 2providing ventilating air and combustion air; a combustion chamber orburner 4 disposed to receive combustion air from the blower 2, andliquid fuel from jet nozzle 6, the burner also containing conventionaligniting spark electrodes (not shown) adjacent the jet orifice of thenozzle; an exhaust stack 8 communicating with the combustion chamber andthrough which exhaust gases are passes to atmosphere; an outer jacket 10defining a heat exchanging passageway between the combustion chamber andthe outer jacket so that ventilating air may be heated in its passagetherethrough; an air outlet 12 through which ventilating air isdischarged into suitable conveying ducts for delivery to the servedspace; and, a discharge air temperature sensing element 14 preferablydisposed in the path of the heated ventilating air and responsive to thedifferential between a selected and a sensed air temperature to controlthe operation of a throttle valve. It is to be understood that while asingle blower means 2 is shown for providing flow of both combustion airand ventilating air, separate blowers of proper capacity may be used foreach of the air flows respectively.

The fuel system includes a tank 16 containing a supply of gasoline or asimilar liquid fuel, a tank outlet line 18 having a fuel filter 20therein and an electrically driven fuel pump 22 for drawing fuel fromthe tank. The direction of fuel flow is indicated by the arrows inFigure 1. The pump 22 is within a housing 24 which also includes aconventional balanced regulating type valve 26. The regulating valve 26is adjusted to open at a predetermined pressure (e.g., psi.) to permitflow of fuel at that pressure into line 28 leading to nozzle 6. Aprimary return or bypass line 30 is connected intermediate the pump 22and regulating valve 26 and includes a two position valve 32 operated bysolenoid 34. With the pump 22 operating and the valve 32 open, all ofthe fuel discharged by the pump flows through the primary bypass line 30and valve 32 back to the tank 16 since the open valve 32 prevents thebuild-up of sufiicient fuel pressure to open regulating valve 26. Itwill thus be apparent that when primary bypass valve 32 is open, no fuelflows to the nozzle through nozzle supply line 28.

A secondary bypass line 36 connects the regulating valve 26 to thereturn line 38 so that when primary bypass valve 32 is closed, andregulating valve 26 is consequently open to pass fuel at thepredetermined pressure into nozzle supply line 28, the excess fuel notrequired to maintain the pressure in the nozzle supply line is bypassedor returned to tank 16.

Burner nozzle 6 is of the conventional bypass type which permits anoperation wherein the rate of fuel discharged into the burner throughnozzle 6 is controlled by throttling fuel flow on the downstream side ofthe nozzle. Such a burner nozzle is illustrated and described in detailin the aforementioned .I-Iubbard U.S. Patent 2,75 8,591 and includes asupply chamber into which fuel from nozzle supply line 28 flows, acombustion jet orifice through which some of the fuel escapes as a sprayor jet into the burner, and a bypass chamber in the nozzle whichreceives the rest of the fuel. With fuel supplied 3 by line 28 at aconstant pressure to the nozzle, part of the fuel will be dischargedthrough the jet orifice into the burner chamber and part of it willbypass to the bypass chamber in, the nozzle.

The nozzle bypass chamber is connected to nozzle return or bypass line40 having a check valve 42 and a throttling valve 44. The check valve 42is provided to prevent residual fuel or an unbalanced pressure betweenthe lines 40 and 28 from causing feedback of fuel to the nozzle afterthe heater has been shut down. The throttle valve 44 serves in normalheater operation to throttle the nozzle bypass line 46 and therebycontrol the rate at which fuel is discharged through the nozzle jetorifice. As will be apparent, the more the nozzle bypass line 40 isthrottled, the more fuel is discharged from the nozzle jet orifice; andconversely, the less the nozzle bypass line is throttled, the less fuelissues through the nozzle jet orifice.

The throttle valve 44 is described and claimed in US. Patent No.2,505,933, issued May 2, 1950* to Clarence A. Aughey et al. andtherefore will be described only briefly. It is controlled, afterselection of a desired air temperature, in response to the discharge airtemperature of the ventilating air sensed by element 14 positioned inthe heater outlet. The temperature responsive element 14 is connected bya capillary tube 46 to the valve 44 for controlling opening and closingof the valve 44 in accordance with sensed temperature conditions at theelement 14. The throttle valve may be adjusted to control fuel flow innormal heater operation to give various selected ventilating airtemperatures by rotating a temperature selection knob 4%, which rotationvaries the compression of an internal valve spring opposing pressure inthe capillary line 46. Assuming the temperature selection knob 48 hasbeen adjusted to give a selected temperature during the normal operationof the heater, a sensed ventilating air temperature exceeding thisselected temperature causes throttle valve 44 to be operated in anopening direction to decrease the fuel discharge rate into the burner.Conversely, when the sensing element 14 senses that the ventilating airtemperature is below the selected value, the throttle valve 44 isoperated in a closing direction to increase the fuel discharge rate intothe burner.

A low fire start valve 50 is connected in a second nozzle return orbypass line, or in other words, in parallel with throttle valve 44between nozzle bypass line 40- and return line 38. The control portionof valve 50 is placed in communication with the nozzle bypass line 30 bymeans of pilot pressure line 52. It will be apparent that since low firestart valve 50 is in parallel with throttle valve 44, it can be operatedto exert control over the rate of fuel discharged from the nozzle jetorifice into the burner. The low fire start valve 50 is arranged to beoperated to exert this control only upon starting combustion in theburner and for a short period thereafter and is controlled to be closedduring normal operation of the heater so that the rate of fueldischarged from the nozzle 6 is then entirely under the control ofthrottle valve 44.

The low fire start valve 59 and its operation will now be explained.Referring to Figures 1 and 2 the valve is a piston operated valve whichincludes an inlet 54 in communication with the nozzle bypass line 46 andan outlet 56 communicating with the return line 38. Fuel flow throughthe valve is controlled by the position of the tapered piston valve 58with respect to its seat. As shown in Figure 2, the valve is closed. Thepiston 69 is slidably mounted for movement in one direction or anotherin piston guide 62. A diaphragm 64 is secured to the piston head anddivides the interior of the valve into a fluid chamber 66 and a springchamber 68. Fluid pressure in chamber 66 urges the piston towards avalve-closed position. The piston is urged in the opposite direction ortowards a valve-open position by an opposing compression spring 70 whichengages the piston guide 62 and piston head. With no fluid in the fluidchamber 66, and consequently no fluid pressure opposing the spring 70,the piston is moved to the left so that the valve is open.

The valve body also includes an inlet 72 to receive fluid from pilotpressure line 52connected to the primary bypass line 30. The inlet 72communicates with the fluid chamber 66 through metering orifice 74 whichhas a needle valve 76 adjustably positioned therein. A check valve 77 inthe fluid chamber outlet 78 is maintained in a closed position by checkspring 80 and serves to close the fluid chamber while fuel from theprimary bypass line is being metered into the fluid chamber throughmetering orifice 74. The needle valve 76 may be adjusted to permit fuelto be metered into the fluid chamber at the proper rate by rotating aconventional'adjusting screw 82.

The specific operation of the low fire start valve 50 in relation toother parts of the fuel system will now be explained. When the solenoidvalve 32 in the primary bypass line is open, fuel flows freely back tothe tank and the pressure in the bypass line and pilot pressure line issufficiently low that a negligible amount of fuel is metered through themetering orifice into the fluid chamber 66. At this time, since thecompression spring 70 alone controls the positioning of the pistonvalve, the spring 71 maintains the piston valve in an open position. However, when the solenoid valve 32 is closed, the increase in fluidpressure in the primary bypass line 30 causes fuel to flow through thepilot pressure line 52, through the metering orifice 74 at a ratedetermined by the position of the metering needle valve 76 and into thefluid chamber 66. As fuel is metered into the fluid chamber, thepressure in the fluid chamber increases gradually until it equals thepressure in the pilot pressure line. As this pressure increases, theforce of the opposing spring 70 is overcome and the piston valve ismoved progressively from an open to a closed position and fuel flowthrough the low fire start valve is stopped. It will be understood thatwhen solenoid valve 3 2 is closed, the increased fuel pressure on thedownstream side of pump 22 causes regulating valve 26 to open so thatfuel flows in nozzle supply line 28.

When the solenoid valve 32 is opened, the fuel pressure in the primarybypass line and pilot pressure line immediately decreases to a valuesubstantially below the fuel pressure in the fluid chamber 66. Thus, thefluid pressure in chamber 66 coupled with the force of compressionspring 70 overcomes the force of the check spring and opens the checkvalve 77 so that the fluid in the fluid chamber is expelled back intothe pilot pressure line 52 through fluid chamber outlet 78; and thepiston 60 is returned to a valve-open position. Thus, subsequent closingvof the solenoid valve 32 will again result in a low fire start of theheater.

As will be apparent, when the low fire start valve 50 is opened topermit the flow of fuel from the nozzle bypass line directlytherethrough to the return line 33, full control of fuel discharge intothe burner by the throttling valve (which will normally be fully closed)is effectively removed. In One method of operation found desirable bysome, the metering needle valve 76 is adjusted so that upon closing ofthe solenoid valve 32, the low fire start valve 54 will be operated froma fully open position to a fully closed position in two minutes. It willbe understood that the low fire start valve is sized so that when it isfully open, the restriction to fuel flow afiorded thereby will cause arate of nozzle discharge sufliciently low to prevent undue thermal shockto combustor and heat exchanger structure and to satisfy other salientconsiderations.

The operation of the solenoid valve 32 by solenoid 34 is controlled bythe circuit shown in Figure 3 and which includes a flame responsivecontrol circuit and certain heater safety control elements. The circuitincludes a source of power 84, a power switch 86 closable to deliverpower to both power line 88 and electronic network 90 such ascommercially available Flameotrol Model 1570-8286 made by Barber ColmanCompany, a thermal overheat switch 92 responsive to an excessivedischarge air temperature, an air flow responsive switch 94, and, aground connection 96. The basic purpose of this circuit is to detectfire or combustion in the burner and to control solenoid 34 in responseto the detected condition. The circuit will be described briefly toexplain how this is accomplished.

The flame scanner 98 which includes a lead sulphide photocell sensitiveto heat is operative, when the electronic network 90 is energized byclosure of switch 86, switch 92 and switch 94, and detection of flame bythe scanner, to energize relay winding 100. Relay winding 100 controlsswitches 102, 104, 106 and 108 shown in Figure 3 in their respectivepositions corresponding to a de-energized winding 100. In other words,with the scanner 98 detecting no flame, and relay winding 100consequently being in a de-energized condition, the switches 102-108 areas shown.

With no flame detected, power is delivered from line 88 through closedswitch 102, and line 110 to a first time delay thermal relay resistanceheater 112. Time delay relay heater 112 is preferably adjusted to delayclosure of its controlled work bar switch 114 for a period of, say, 30seconds. This time delay is so that solenoid 34 cannot be energized andthus fuel cannot be supplied to the burner until the combustion airblower has operated for 30 seconds; thus insuring that any residualexplosive mixture of gas and air in the combustion chamber will bepurged. When switch 114 closes after 30 seconds, power is made availableat one side of normally open pushbutton switch 116. When switch 116 ismanually closed, there occurs energization of a circuit including line118, closed switch 106, line 120, a second time delay thermal relayresistance heater 122, line 124, relay winding 126 and line 128. Whenrelay winding 126 is energized, its controlled switches 130 and 132 areactuated to a closed position. Warp bar switch 134, which is controlledby second time delay relay heater 122 operates from a closed to an openposition within two to five seconds after second time delay relay heater122 has been energized. Thus, when push-button switch 116 is closed andsecond time delay relay heater 122 and relay 126 are energized, theclosure of switch 132 resulting from energization of relay 126 completesa first solenoid energizing circuit which energizes solenoid 3-4 by wayof power line 88, closed switch 132, line 136, closed second time delayswitch 134, and line 138 to the solenoid 34.

When solenoid 34 is energized, its controlled valve 32 (Figure l) closesand fuel passes to the nozzle 6 and is discharged into the burner asexplained heretofore. The fuel ignition means, which has been energizedthrough a separate circuit, ignites the fuel issuing from nozzle 6 andflame scanner 98 immediately detects the fire in the burner. Whenscanner 98 detects the fire, relay winding 100 is energized by operationof electronic network 90, and switches 102-108 operate to positionsopposite to those shown in Figure 3. As mentioned, from two to fiveseconds after second time delay relay heater 122 has been energized, itcauses its controlled switch 134 to open and the first solenoidenergizing circuit is thereby opened. However, assuming flame has beenestablished, when the second time delay switch 134 operates to open thefirst solenoid energizing circuit, an alternate solenoid energizingcircuit including switch 132, line 136, line 140, closed switch 108(closed by energization of relay winding 100), and line 138 has beenestablished.

The function of switch 130 is to provide an alternate path formaintaining its associated relay winding 126 energized throughcombustion-closed switch 104, line 142, closed switch 130 and line 144connecting switch 130 to second time delay relay heater 122 when flamehas been detected and push button switch 116 released. It will be notedthat when switch 102 opens, due to establishment of flame, the firsttime delay relay heater ll2 is de-energized so that it will have anopportunity to cool and to reset itself for providing a thirty seconddelay when again energized. If flame should fail after having beenestablished, the scanner 98 will detect this, relay 100 will bede-energized, and switches 102-108 will be actuated to the positionsshown in Figure 3 so that the alternate solenoid energizing circuitwillbe de-energized by opening of switch'108.

An overheat condition as sensed by overheat switch 92, or failure ofcombustion or ventilation air as detected by switch 94, will open thecircuit to ground 96 and result in de-energization of the entirecircuit.

Operation Before the heater is started, the throttling valve 44 isadjusted by temperature selection knob 48 to control fuel flow in normaloperation to give a desired temperature. An ignition switch is closed toprovide an igniting spark at the jet orifice of the nozzle 6. Theignition power may suitably be provided by a magneto driven from a primemover furnishing ventilation and combustionair. The fuel pump 22 isenergized to draw fuel from the tank 16 and force it through the primarybypass line 30 and open solenoid valve 32 back to the tank. Withcombustion and ventilation air flowing from the prime mover operatedblower, the air flow responsive switch 94 is closed. Since solenoidvalve 32 is open no fuel is supplied to the burner since regulatingvalve 26 will not open until the pressure of fuel at its inlet isincreased to at least a certain value by closure of primary bypass line.

Power switch 86 is then closed to energize first time delay relay 112.After thirty seconds, first time delay switch 114 closes and solenoid 34is energized and valve 32 closed by closing the push-button switch 116.When solenoid valve 32 closes the primary bypass line, the increasedfuel pressure at regulating valve 26 opens it and fuel flows at apredetermined pressure in nozzle line 28 to the nozzle 6. As has beenexplained before, the low fire start valve 50 is biased to an openposition at this time by spring 70 so that a minimum or relatively lowquantity of fuel is discharged into the burner and ignited. The throttlevalve 44 is substantially closed at this time since the ventilating airtemperature sensed by element 14 is considerably below the desiredtemperature. When ignitionis detected by the flame scanner 98, relay 100is energized and the switches 102-108 are operated to their alternatepositions to provide the closed alternate solenoid energizing circuitwhen the second time delay switch 134 opens the first solenoidenergizing circuit.

With solenoid valve 32 closed, the substantial fluid pressure in pilotpressure line 52 causes fuel to be metered into the fluid chamber 66 ofthe low fire start valve 50. This causes the piston valve to be operatedprogressively towards a closed position so that after approximately twominutes, the low fire start valve is closed. This progressive closing ofthe low fire start valve will result of course in a progressive increasein fuel discharged into the burner and a progressive restoration of fuelcontrol to the throttle valve 44.

Assuming the heat exchanger of the heater was relatively cold when theburner was started, the ventilating air discharge temperature as sensedby element 14 will be relatively low and will progressively increase asthe heat exchanger becomes warmer. As the temperature at the sensingelement 14 increases, the throttle valve 44 will be operated from afully closed position towards a position corresponding to the properopening .for the desired ventilating air temperature. Thereafter, if theventilating air discharge temperature sensed by element 14 rises aboveor falls below the selected temperature due to fortuitous conditions,the throttle valve 44 will be modulated in an opening or closingdirection respectively to restore the selected temperature. During thisnormal operation, the low fire start valve is maintained in a closedposition by the fluid pressure opposing the compressed spring in thisvalve.

If flame should fail in the burner, this will be detected by the scanner98, and relay 100 will be de-energized causing operation of switches102408 to a position which tie-energizes solenoid 34. Thus, the solenoidvalve 32 and primary bypass line 30 will be opened for bypassing fuelback to the tank and terminating flow into the nozzle supply line 28.When this happens, the low fire start valve 50 will reset itself to givea low fire start when burner operation is re-initiated.

It will be seen then that, in addition to the low fire start feature,the system: prevents fuel flow to the burner until the elapse of apredetermined time after combustion air flow is initiated to insure thatthe burner is purged; requires that unless fire is established within ashort period, such as 2-5 seconds, after fuel flow to the burner begins,fuel flow to the burner is terminated; terminates fuel flow to theburner if at any time during heater operation the fire fails, anoverheat occurs or combustion or ventilating air fails; and,automatically resets itself to give a low fire start, purging and burnerfuel flow termination upon restarting after fire failure or terminationof heater operation.

Having described my invention, I claim:

1. A fuel control system for a fluid fuel burning air heater,comprising: a burner; a main fuel line connected to supplyfuel to .saidburner; fuel pump means for supplying fuel to said main fuel line; aprimary bypass line connected to said main fuel line; a two-positionvalve in said primary bypass line; switch means adapted to be closedupon the need for heat; electrical means operative in response toclosure of said switch means to close said two-position valve; meansincluding a bypass line from said burner and a throttle valve thereinfor normally controlling the supply of fuel to said burner in responseto a differential between a sensed air temperature produced by saidburner and a selected air temperature; a valve in parallel with saidthrottle valve operative, when open, to remove control of fuel flow fromsaid throttle valve; and fuel pressure responsive means operative toprogressively close said parallel valve and thereby progressivelyrestore control of fuel flow to said throttle valve in response to anincrease in the fluid pressure condition in said primary bypass linecorresponding to closure of said two-position valve.

2. A fuel control system for a fluid fuel burning air heater,comprising: a burner having a jet fuel nozzle; a nozzle supply lineconnected to supply fuel to said nozzle; pump means for supplying fuelto said supply line; a nozzle bypass line for bypassing fuel notdischarged from said nozzle into said burner; a discharge airtemperature responsive throttle valve in said nozzle bypass lineoperative to normally control 'nozzle discharge in response todifferentials between a selected air temperature and a sensed dischargeair temperature produced by said burner; a primary bypass line connectedto said supply line; a valve in said primary bypass line operative whenopen to bypass fuel and prevent flow in said nozzle supply line, andwhen closed to prevent fuel flow in said primary bypass line; means forclosing said primary bypass line valve upon the need for heat; a valvein parallel with said throttling valve in said nozzle bypass line; andmeans responsive to the fuel flow condition in said primary bypass linefor operating said parallel valve from an open position progressively toa closed position in response to a change from a full flow condition toa noflow condition in said primary bypass line.

3. The system of claim 2 including: means biasing said parallel valvetowards an open position.

4. The system of claim 2 wherein: said primary bypass line valve is asolenoid operated valve biased to an open position; and initiatingcircuit means including first switch means adapted to be closed upon theneed for heat 8 are provided for energizing said solenoid operated valveto operate it to a closed position to terminate fuel flow in saidprimary bypass line when combustion is to be initiated.

5. The system of claim 4 including: solenoid holding circuit meansincluding second switch means adapted to be held closed to maintain saidsolenoid energized in response to a combustion condition in said burner;means responsive after an initial time delay to a non-combustioncondition in said burner for opening said second switch means andtie-energizing said solenoid operated valve whereby a full flowcondition is restored in said primary bypass line.

6. The system of claim 5 including: overheat switch means in saidcircuit means for de-energizing said solenoid operated valve in responseto a sensed overheat condition.

7. The system of claim 6 including: air flow responsive switch means forde-energizing said solenoid operated valve in response to a sensed airflow below a predetermined safe value.

8. In a fuel control system for a fluid fuel burner: a fuel burner; fuelsupply means including a fuel pump, a burner supply line, and aregulating valve intermediate said pump and said burner supply lineadapted to open when the fuel pressure on the upstream side of saidregulating valve equals or exceeds a predetermined value; fuel returnline means including a first burner fuel return line having aburner-produced air temperature responsive throttle valve for normallycontrolling fuel discharge into said burner, a second burner fuel returnline in parallel with said first burner fuel return line and having anopenly-biased low fire start valve therein, and a primary fuel returnline connected intermediate said pump and said regulating valve andhaving a two-position openly-biased valve therein; switch means adaptedto be closed upon the need for heat; electrical means operative inresponse to closure of said switch means to close said two-positionvalve; and means for operating said low fire start valve progressivelyfrom an open to a closed position in response to closure of said primaryfuel return line valve causing an increase in fuel pressure on theupstream side of said regulating valve to a value equaling or exceedingsaid predetermined value.

9. In a fuel control system as specified in claim 8: means responsive tocombustion in said burner for maintaining said primary fuel return linevalve closed by maintaining said electrical means energized whilecombustion continues, and for opening said primary fuel return linevalve by de-energizing said electrical means when combustion ceases.

-10. A fuel control system as specified in claim 8 wherein: said lowfire start valve operating means includes a pilot pressure lineconnecting the upstream side of said primary fuel return line valve withsaid low fire start valve; and said low fire start valve includes afluid pressure chamber having a restricted inlet adapted to receivefluid at a metered rate through said pilot pressure line whereby, uponan abrupt increase in fluid pressure in said pilot pressure line, thefluid pressure in said fluid pressure chamber increases at a relativelyslower rate.

11. A fuel control system as specified in claim 10 wherein: said lowfire start vaive fluid pressure chamber includes a normally closed fluidrelief outlet operative, in response to opening of said primary returnline valve and a consequent abruptly decreased pressure in said pilotpressure line, to open for exhausting the fluid in said fluid chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,178,672 Perkins Nov. 7, 1939 2,179,846 Finnigan Nov. 14, 19392,263,833 Aldrich Nov. 25, 1941 2,758,591 Hubbard Aug. 14, 1956

